Kinetic and Thermodynamic Aspects of the Bainite Reaction in a Silicon Steel
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KINETIC AND THEBY1ODYN'AMIC ASPECTS OF THE BAINITE REACTION IN A SILICON STEEL G. PAPADIMITRIOU AND J.M.R. GENIN* Department of Mining and Metallurgy, National Technical University of Athens, 42 Patission Street, Athens, Gr 147 *Departement des sciences et techniques des Mat~riaux, Institut des Sciences de l'Ingenieru, Universite de Nancy 1, Vandoeuvre - Nancy, France 54500 ABSTRACT
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The bainite reaction in an Fe - 3.85 wt pct Si - 0.9 wt pct C steel 0 studied by several experimental techniques in the range of 250-450 C.
The high silicon content prevents the formation of cementite, so that the reaction is separated to two clearly distinct stages. In the primary stage ferrite forms alone, except at temperatures lower than 0 310 C where some carbides precipitate in it, and austenite becomes enriched in carbon. In the secondary stage occurring only above 400*C, the enriched austenite decomposes to ferrite and an unknown silicon carbide. The microstructure, the enrichment of the austenite and the overall reaction kinetics of the two stages are studied and are found to be consistent with a displacive mechanism of the bainite reaction. A tentative model, accounting for the competition of shear and diffusion, is proposed in order to fit our experimental data. INTRODUCTION The formation of bainite remains one of the most controversial transformations in steels. The bainite occurs as the product of the austenite decomposition in the intermediate range of temperatures, between the diffusional peralite reaction and the martensitic shear transformation. From a morphological point of view the product defined as bainite is a nonlammelar aggregate of ferrite and carbides [1]. Bainite possesses its own C curve [2] and it is distinguished in upper and lower bainite. The bainitic structure formed in the upper range of temperatures has a feathery morphology [3] and consists of ferritic laths or plates and interlath carbide precipitates [4]. The bainitic ferrite formed in the lower range of temperatures is lenticular and finer and contains intralath carbides, with a habit plane forming 600 with the axis of the plate [5]. The bainite possesses both diffusional and shear characteristics, cuasing conflict about the mode of the y/a transformation, whether it takes place by a reconstructive or displacive mechanism. Aaronson and Kinsman [1] support the view of a purely diffusional mechanism, where the bainitic ferrite grows by the propagation of ledges. The growth is controlled by the metastable n/y equilibrium at the austeniteferrite interface. Hehemann [1] supports the formation of the bainitic ferrite by a shear mechanism. The bainitic plate is composed by a large number of subunits grown martensitically [f] and initially formed by a sympathetic nucleation process [1,4,6]. The ferrite subunits are formed with a carbon content greater than that corresponding to the a/a + y equilibrium [1,4] and with a rate higher than that corresponding to volume diffusion. The growth of the Mat. Res. Soc.Symp. Proc. Vol. 21 (1984) Q Elsevier Science
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